Hydrogen is considered an attractive alternative to fossil fuels, particularly, over the past decade and as the most lightweight fuel, it has the potential to provide a supply of clean, reliable and affordable energy to meet growing demands. However, there have been problems associated with the commercialization of hydrogen fuel, such as, the inability to economically generate clean and pure hydrogen combined with cost-effective storage and distribution.
One approach to hydrogen production involves splitting water using electricity, i.e., water electrolysis. In this approach, electric current is passed through water which splits the water into hydrogen and oxygen. In certain embodiments, electricity-induced splitting of water is conducted using alkaline, neutral or acidic pH mediated proton exchange membrane (PEM)-based water electrolysis.
Electricity-induced splitting of water using alkaline, neutral or acidic pH mediated proton exchange membrane (PEM)-based water electrolysis is potentially an advantageous approach for hydrogen production because it offers essentially no greenhouse gas emission or toxic byproducts, particularly if the electric current is generated using renewable energy sources such as wind, solar, geothermal, hydropower, or nuclear energy sources.
In principle, PEM-based water electrolyzers are built-up similar to PEM fuel cells, however, they work in a different manner. During PEM fuel cell operation, oxygen reduction takes place at the cathode and hydrogen oxidation occurs at the anode of the fuel cell. In summary, water and electrical current is produced. In a PEM-based water electrolyzer, the current flow and the electrodes are reversed and water decomposition takes place. Oxygen evolution occurs at the anode (abbreviated “OER”, i.e., oxygen evolution reaction) and reduction of protons (W) which travel through the membrane occurs at the cathode. As a result, water is decomposed into hydrogen and oxygen by means of the electric current.
The capital costs for current electrolyzer technology is a significant barrier to hydrogen production. Thus, progress in the production of hydrogen fuel by water electrolysis has been slow due to prohibitive costs of current electrolyzer technology. The high capital costs of current electrolyzers is due to one or more of the following: deployment of expensive noble metal-based electro-catalysts (e.g., IrO2, RuO2, Pt), use of relatively small and comparatively low efficiency systems, customized power electronics and labor intensive fabrication.
Rutile-type noble metal oxides, such as IrO2 and RuO2, are well known for OER anode electrode catalysts in alkaline and PEM-based water electrolysis. However, the anodic over-potential and the cell resistance in electrolysis contribute to a majority of the losses observed in catalytic performance. In addition, IrO2 and RuO2 electro-catalysts undergo electrochemical or mechanical degradation under extreme and highly corrosive electrochemical environments prevalent in acid assisted water electrolysis which reduces the performance with time and diminishes the service life of the electrode during OER.
The use of non-precious metal catalysts for electrodes in PEM-based electrolysis cells would decrease the overall costs of hydrogen production. It is, however, unlikely that a non-noble metal catalyst would be effective to produce hydrogen in a PEM-based electrolysis system. Although, it is contemplated that a decrease in the noble metal loading (e.g., and the use of a purely non-noble metal in combination with the noble metal) may produce an electrode catalyst capable of demonstrating activity and stability comparable to a pure noble metal catalyst for a reduced cost.
There are a limited number of materials that are known to exhibit the desired electrical conductivity, as well as the electrochemical, structural, physical and chemical stability needed in the 1.8 to 2.0 V potential window and therefore, would be suitable for use in producing a non-noble metal-containing catalyst electrode for PEM-based water electrolysis. Thus, it is an object of this invention to develop highly efficient and stable electro-catalyst compositions which include a combination of non-noble metal and noble metal oxides to significantly reduce the amount expensive noble metal required for use as anode electrodes in PEM-based water electrolysis.